The present invention relates generally to vibration sensing apparatus, and more particularly to a means for sensing vibration in a non-linear optical crystal using laser beams.
A great variety of means and methods for sensing vibration have been developed. An example is found in U.S. Pat. No. 4,471,659, issued to Udd et al. Some of these have taken advantage of the properties of crystals, such as that disclosed in U.S. Pat. No. 4,900,919, issued to Twerdochlib. Other methods for detecting vibration have utilized coherent light beams, such as the method described in U.S. Pat. No. 4,466,738, issued to Huang et al.
A heretofore essentially unrelated field has been that concerning the properties of wave mixing within a photorefractive crystal. Various adaptions and applications concerning this property have been devised, such as that disclosed in U.S. Pat. No. 4,703,992, issued to Yeh. Wave mixing occurs when, in a crystal, two intersecting laser beams form an intensity interference pattern which creates a refractive index grating via the electro-optic effect in the photorefractive crystal. The grating is a periodic variation in the index of refraction of the crystal.
It happens that, in conventional two wave-mixing, vibrations in the laser beam and the crystal cause transient variation in the effective gain, which results in fading and distortion in an output signal. Therefore, it has been thought that no useful signal could be derived from the wave mixing property inherent in photorefractive crystals which could be useful for accurately and reliably providing an output signal modulated according to vibrations of the crystal.
Clearly, if the wave mixing properties of a crystal could be utilized to provide an output modulated by vibration of the crystal, then there would be attendant advantages. Among the advantages would be the inherent precision of measurement of the laser interferometric principles involved, and the ability to produce a small and rugged vibration sensor the output of which could be transmitted through optical fibers to detector apparatus. However, the instabilities associated with wave mixing have reduced the usefulness of the application of this principle such that, to the inventor's knowledge, nor prior art method has been developed to sense vibrations in a crystal using wave mixing principles.
No prior art, to the inventor's knowledge, has successfully provided a stable laser output modulated by vibrations in a crystal, and all previous applications have resulted in output signals which suffer from unwanted distortion and fading.